For optimum visibility strong contrast is employed between the lettering or indicia and background on a magnetic compass. Using white indicia on black is a common combination as is typical with automotive dials, or the reverse with black indicia on a white background. In dark environments such instruments are illuminated with white light from either an incandescent bulb or a light emitting diode (LED). Using colour has also been found to be effective in producing even better contrast. A combination of red and white is common and employed with important road signs such as stop signs with white lettering on a red background and also important safety labels (such as fire-hose cabinets). However where levels of illumination are low the physiology of the eye makes other colour combinations easier to read. For example it has been found that green and black offers a highly visible combination in dark environments.
It is desirable to have instruments that can offer different colour combinations depending on whether it is night or day. Hitherto this has not been achieved with for example coloured indicia illuminated with white light or black and white indicia illuminated with coloured light.